1. Field of Invention
The present invention relates to a method of driving a plasma display panel for use as a low-profile and lightweight display device with a large screen.
2. Description of the Related Art
An alternating-plane discharge-type panel typified by a plasma display panel (hereinafter, simply referred to as “panel”) is formed with a large number of discharge cells between a front plate and a rear plate, which are disposed opposing each other. The front plate is formed with, on a front glass substrate, a plurality of display electrodes configuring a plurality of pairs of scanning electrode and sustain electrode in a parallel manner, and to cover such display electrodes, a dielectric layer and a protection layer are formed. The rear plate is formed with a plurality of parallel data electrodes on a rear glass substrate, a dielectric layer to cover those, and a plurality of partition walls thereon in parallel to the data electrodes. A fluorescent layer is each formed to the surface of the dielectric layer and the side surfaces of the partition walls. In such a manner that the display electrodes spatially intersect with the data electrodes, the front plate and the rear plate are disposed opposing each other and sealed. The inner discharge space is filled with a discharge gas. Herein, a discharge cell is formed to any portion formed by the opposing display electrode and data electrode. With a panel configured as such, gas discharge in the respective discharge cells generates ultraviolet rays, and the ultraviolet rays excite the fluorescent layers of RGB colors for light emission so that the color display is made.
As a method of driving the panel, a subfield method is popular, i.e., a field period is divided into a plurality of subfields (hereinafter, simply referred to as “SFs”), and then the subfields are combined together for light emission so that the luminance display is made. The subfield method includes a driving method with which the contrast ratio is improved by suppressing the increase of black luminance through reduction, to a minimum, of light emission not affecting the luminance display.
Such a driving method is described below. FIG. 11 is an operation drive timing chart showing a conventional plasma display panel driving method. Each SF includes an initialization period, a writing period, and a sustain period. In the initialization period, an initialization operation of either an every-cell initialization operation or a selective initialization operation is performed. With the every-cell initialization operation, every discharge cell in charge of image display is made to perform initial discharge, and with the selective initialization operation, any discharge cell through with sustain discharge in the immediately-preceding SF is made to selectively perform initial discharge. With the drive waveform of FIG. 11, the every-cell initialization operation is performed in the initialization period of a 1SF, and in the initialization periods of a 2SF to the last SF, the selective initialization operation is performed.
First of all, in the initialization period of the 1SF, every discharge cell goes through initial discharge all at once, thereby deleting the previous histories of a wall charge on the respective discharge cells, and forming any needed wall charge for the subsequent writing operation. Not only that, there is a function of generating priming (initiating agent for discharge=exciting particles) for reducing a discharge delay, and causing writing discharge with stability. Every data electrode and every sustain electrode are maintained at 0 (ground potential), and every scanning electrode is applied with a lamp voltage that gently increases from a voltage Vp of a discharge start voltage or lower to a voltage Vr exceeding the discharge start voltage. This causes weak discharge in every discharge cell, stores a positive wall charge on the sustain electrodes and the data electrodes, and stores a negative wall charge on the scanning electrodes. Thereafter, every sustain electrode is maintained at a voltage Vh, and every scanning electrode is applied with a lamp voltage that gently decreases from a voltage Vg to a voltage Va. This causes weak discharge in every discharge cell, and weakens the wall charge stored on the electrodes. With such an every-cell initialization operation, the voltage in the discharge cells is put in the state closer to the discharge start voltage. Herein, the period in which the voltage increases from the voltage Vp to the voltage Vr is referred to as an ascending lamp period, and the period in which the voltage decreases from the voltage Vg to the voltage Va is referred to as a descending lamp period.
In the writing period of the 1SF, the scanning electrodes are sequentially applied with a scanning pulse, and the data electrodes are applied with a writing pulse corresponding to a video signal for display. Through such pulse application, writing discharge is caused selectively between the scanning electrodes and the data electrodes in any displaying discharge cell (display cell), and a wall charge is selectively formed. In the sustain period subsequent to the writing period, a sustain pulse is applied between the scanning electrodes and the sustain electrodes for a predetermined number of times, depending on the luminance weight, and in any discharge cell through with wall charge formation by the writing discharge, sustain discharge is selectively caused for light emission. With such light emission, the video is displayed.
In the initialization period of the 2SF, every sustain electrode is maintained at the voltage Vh, every data electrode is maintained at 0, and every scanning electrode is applied with a lamp voltage that gently decreases from a voltage Vb to the voltage Va. During when this lamp voltage decreases, weak discharge is caused in the discharge cell (s) through with the sustain discharge in the immediately-preceding sustain period (sustain period of the 1SF) so that the wall charge formed on the electrodes is weakened, and the voltage in the discharge cells is put in the state closer to the discharge start voltage. On the other hand, in the discharge cell(s) not through with the writing discharge and the sustain discharge in the 1SF, no weak discharge is caused in the initialization period of the 2SF, and the discharge cell(s) remain in the wall charge state after the initialization period is through in the 1SF.
As to the writing period and the sustain period of the 2SF, by waveform application similarly to the 1SF, sustain discharge is caused in any discharge cell corresponding to a video signal. As to the 3SF to the last SF, by drive waveform application to the electrodes similarly to the 2SF, the video display is made.
As such, for correct video display, it is important to perform selective writing discharge with reliability in a writing period, and for the purpose, it becomes important to perform, with reliability, an initialization operation to be ready for the writing discharge. Note here that the details of such a technology is disclosed in Japanese Patent Unexamined Publication NO. 2000-242224.
The issue here is that, in the initialization period of the 1SF of FIG. 11, the initial discharge must cause the scanning electrodes to each serve as an anode, and cause the sustain electrodes and the data electrodes to each serve as a cathode. However, because the data electrodes are each coated thereon with a fluorescent element whose secondary electron emission coefficient is low, the discharge delay is easily increased for the initial discharge with the data electrodes each serving as a cathode. What is more, recently, the study is under way to increase the light emission efficiency by increasing the partial pressure of xenon, which is a discharge gas filled in the panel. However, increasing the partial pressure of xenon, as such, results in a tendency of increasing the discharge delay of the initial discharge. Moreover, if the panel is used for a long length of time, the discharge delay is increased for the discharge cells. If the discharge delay is increased for the discharge cells as such, the initial discharge becomes unstable, and in the discharge cells with the longer discharge delay, the initial discharge that is supposed to be less intense in the ascending lamp period is sometimes increased in intensity. If this is the case, the initial discharge to be caused in the descending lamp period is also increased in intensity.
Also with the longer discharge delay, the writing discharge to be caused only to the display cells in a writing period is made unstable. The wall charge is thus not sufficiently formed, and there may be a case of failing in sustain discharge in the subsequent sustain period. With this being the case, the scanning electrodes are each stored thereon with a positive wall charge, and the sustain electrodes are each stored thereon with a negative wall charge. With the electrodes being in such states, the operation moves to the subsequent initialization period, and in the next initialization period for the every-cell initialization operation (initialization period of the 1SF), the resulting initial discharge caused in the ascending lamp period will be increased in intensity. As a result, the initial discharge to be caused in the descending lamp period is also increased in intensity.
As such, if the initial discharge is increased in intensity in the initialization period of the 1SF for the every-cell initialization operation, the scanning electrodes, as a result, store thereon too much positive wall charge by the time when the initialization period is through. In the discharge cells, even if no writing operation is executed in the subsequent writing period, the sustain discharge may be caused in the sustain period. That is, the discharge cells other than the display cells are illuminated, thereby resulting in erroneous discharge. Furthermore, because the intensity of such erroneous discharge is increased with a larger number of sustain pulses, the erroneous discharge is considerably conspicuous in the SFs with the larger luminance weight.
As such, the erroneous discharge occurring in the conventional drive method is very conspicuous, thereby greatly degrading the display quality.